atmometer

thumb|upright|Spherical porous cup atmometer An atmometer or evaporimeter is a scientific instrument used for measuring the rate of water evaporation from a wet surface to the atmosphere. Atmometers are mainly used by farmers and growers to measure evapotranspiration (ET) rates of crops at any field location. Evapotranspiration is a measure of all of the water that evaporates from land surfaces plus the water that transpires from plant surfaces. Based on the amount of water that does evaporate and transpire, the user can water crops correspondingly, which results in less water use and possibly increased crop yields. Companies that currently sell atmometers include C&M Meteorological Supply and Calsense.

== Design == An atmometer consists of a porous, ceramic plate connected to a water reservoir by a glass or plastic tube. The device stands around tall with a diameter of . Water is drawn from the water reservoir through the tube to wet the plate. As the water on the plate evaporates, more water is drawn from the reservoir to re-wet the plate. A canvas cover made of Gore-Tex is placed over the plate to prevent anything from getting in. The canvas cover is important because it simulates the amount of solar radiation a plant absorbs under certain weather conditions and controls the rate of evaporation. Different types of canvas covers simulate different amounts of evapotranspiration rates that various plant surfaces would undergo. For example, alfalfa ET rates are estimated using the No. 54 green canvas cover, while grass ET rates are estimated using the No. 30 green canvas cover. A membrane between the plate and canvas cover prevents rain water from wetting the gauge yet allows water vapor to escape. A gauge that runs up the side of the atmometer measures the level of water (in inches) in the reservoir, which shows how much water has evaporated.